Traditional string instruments usually have a resonance box portion for increasing volume or for changing tone quality. However, in order to facilitate sound reception and music recording, a variety of electronic string instruments have been developed. The electronic string instruments omit the structure of the resonance box, employ a pickup device for directly receiving sounds generated by strings, and transmit electrical sound signals transformed thereby to a speaker system for output using wired or wireless data transmission techniques. However, direct sound reception from the strings may lead to loss of unique tone quality of resonance resulting from the traditional resonance box structure. In addition, since the string instruments only have limited positions for installation of conventional pickup devices, different pickup devices that are designed for different audio frequency ranges (e.g., low, middle, and high audio frequency ranges) may be unable to be installed on the string instruments at the same time.